Optical element and optical apparatus

ABSTRACT

An optical element including a first layer (011) made of a medium having optical anisotropy, wherein a difference between refractive indexes nh and nl (nh&gt;nl) at a central wavelength λ for first and second polarized lights which enter the optical element and whose polarization directions are different from each other is at least 0.1, and wherein conditions (nt1−nl)·(nl−nt2)≰0 and nt1&lt;nl are satisfied, where nt1 and nt2 denote refractive indexes of a second layer (012) and a third layer (013) optically adjacent to the first layer in both sides of the first layer and made of isotropic media at the central wavelength.

BACKGROUND OF THE INVENTION

The present invention relates to an optical element using an opticalanisotropic medium, and an optical apparatus such as a projector whichuses the optical element.

The optical element that uses the optical anisotropy has been in wideuse for polarization control, beam separation, or the like. For example,the optical element is used as a phase plate represented by a λ/4 plateor a λ/2 plate, a polarization plate represented by a polarizationseparation element, or an optical low-pass filter.

The optical anisotropy means a nature of variance of refractive indexesdepending on a vibration direction of an incident polarized light. Theuse of this nature enables variance of behaviors depending onpolarization directions even in the case of lights entering from thesame direction.

Materials having such optical anisotropy include a crystal material suchas a crystal or a limestone, a liquid crystal material, and an organicmaterial such as plastic or a high molecule. A degree of opticalanisotropy of such a material is represented by a refractive index withrespect to a polarization direction.

Japanese Patent Laid-Open Nos. 2004-139001 and 2007-156441 disclose, asa method for obtaining optical anisotropy, methods which use structuralanisotropy based on structures smaller than a wavelength of a used light(hereinafter referred to as a used wavelength).

In the structure smaller than the used wavelength, a light is known tobehave like a homogeneous medium without being able to directlyrecognize the structure. In this case, the light exhibits a refractiveindex compliant with a filling rate. The refractive index can beobtained by a method called an effective refractive index method. Anature of variance of refractive indexes depending on polarizationdirections according to the filling rate of the structure is calledstructural anisotropy. Optionally setting a filling rate of thestructure enables adjustment of a refractive index. The use ofstructural anisotropy enables an increase of a difference of refractiveindexes depending on polarization directions as compared with a normaloptical anisotropic material. Thus, a thickness for obtaining desiredbirefringence characteristics can be reduced.

Japanese Patent Laid-Open No. 2004-139001 discloses a phase plate whichuses structural anisotropy. In the phase plate, by using the capabilityof the structural anisotropy to adjust the refractive index, a pluralityof structural anisotropic layers (periodic structures) are combined tosuppress changes in phase difference characteristics caused bywavelengths.

Japanese Patent Laid-Open No. 2007-156441 discloses an optical elementwhich includes a structural anisotropic layer of a plane normaldirection formed in one surface of a substrate, and a structuralanisotropic layer of an in-plane direction in the other surface. A phasecompensation plate is obtained by adjusting a refractive index based onthe structural anisotropy of each surface and combining the refractiveindexes. An antireflection coating is inserted to provide anantireflective function.

Japanese Patent Laid-Open No. 2004-139001 discloses an example where amaterial of a low refractive index having similar periodicity isdisposed on a structure anisotropic layer using a medium of a highrefractive index. Thus, since the material of a refractive index lowerthan that of each structural anisotropic layer is stacked thereon,reflection on the surface is suppressed to a certain extent. With thisconfiguration, however, antireflective performance is insufficient.

In Japanese Patent Laid-Open No. 2007-156441, an antireflection coatingis provided to a structural anisotropic layer. However, no configurationnecessary for exhibiting an antireflective function is disclosed.

When the material of large refractive index variance depending onpolarization directions is used, reflection-transmission characteristicsgreatly vary with respect to the polarization directions. Even whenantireflective coating is provided, because of the large variance ofrefractive indexes, optimization of characteristics for eachpolarization direction is difficult.

SUMMARY OF THE INVENTION

The present invention provides an optical element which uses opticalanisotropy and exhibits sufficient antireflective performance, and anoptical apparatus which uses the optical element.

An optical element as one aspect of the present invention includes afirst layer made of a medium having optical anisotropy. A differencebetween refractive indexes n_(h) and n_(l) (n_(h)>n_(l)) at a centralwavelength λ for first and second polarized lights which enter theoptical element and whose polarization directions are different fromeach other is at least 0.1. The following conditions (1) or (2) aresatisfied, where n_(t1) and n_(t2) denote refractive indexes of secondand third layers optically adjacent to the first layer in both sides ofthe first layer and made of isotropic media at the central wavelength:(n _(t1) −n _(l))·(n _(l) −n _(t2))≦0n _(t1) <n _(l)  (1)(n _(t1) −n _(h))·(n _(h) −n _(t2))≦0n _(t1) >n _(h)  (2)

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a basic configuration of an opticalelement which is an embodiment of the present invention.

FIGS. 2A and 2B are views each showing a refractive index and an opticalfilm thickness of the optical element of the embodiment.

FIG. 3 is a view showing a refractive index and an optical filmthickness of only a substrate of the optical element.

FIG. 4 is a view showing a refractive index structure when a thin filmof a lower refractive index is formed on the substrate.

FIG. 5 is a view showing a refractive index structure when a thin filmof a higher refractive index is formed on the substrate.

FIG. 6 is a schematic view of the optical element of the embodimentwhich includes a structural anisotropic layer.

FIG. 7 is a view showing reflectance characteristics of an opticalelement of Embodiment 1.

FIG. 8 is a view showing reflectance characteristics of an opticalelement of Embodiment 2.

FIG. 9 is a view showing reflectance characteristics of an opticalelement of Embodiment 3.

FIG. 10 is a view showing reflectance characteristics of an opticalelement of Embodiment 4.

FIG. 11 is a view showing reflectance characteristics of an opticalelement of Embodiment 5.

FIG. 12 is a view showing reflectance characteristics of an opticalelement of Comparative Example 1.

FIG. 13 is a view showing reflectance characteristics of the opticalelement of Comparative Example 1.

FIG. 14 is a view showing a configuration of a liquid crystal projectorwhich uses the optical elements of any of Embodiments 1 to 5.

FIG. 15 is a view showing a configuration of an optical pickup apparatuswhich uses the optical element of any of Embodiments 1 to 5.

FIG. 16 is a sectional view showing an optical element which includesonly a substrate.

FIG. 17 is a sectional view showing an optical element which includes athin film formed on a substrate.

FIGS. 18A and 18B are views each illustrating a refractive indexstructure of an optical element which includes an optical anisotropiclayer and a thin film stacked on a substrate.

FIG. 19 is a sectional view showing an optical element of an embodimentwhere an insertion layer is added to the basic configuration of FIG. 1.

FIGS. 20A and 20B are views each illustrating a refractive indexstructure of the optical element of FIG. 19.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the present invention will be described belowwith reference to the accompanied drawings.

First, features common among optical elements of embodiments will bedescribed before description of specific embodiments. FIG. 1 illustratesa basic configuration example of an optical element.

Reference numeral 011 denotes a first layer formed by a medium ofoptical anisotropy. Reference numerals 012 and 013 denote second andthird layers constituted of homogeneous isotropic thin films and formedin both sides of the first layer 011 adjacently to the first layer 011.Reference numeral 014 denotes a substrate, and reference numeral 015denotes an antireflective layer (antireflection coating) constituted ofthe first to third layers 011 to 013.

In the description below, an optical anisotropic medium for forming thefirst layer 011 is a uniaxial anisotropic material having an opticalaxis in a y-axis direction in the drawing. A refractive index of thefirst layer 011 with respect to a central wavelength (hereinafterreferred to as a used central wavelength) of a light (first polarizedlight where x and z directions are polarization directions) which entersthe optical element and vibrates in the x and z directions is defined asn_(h). A refractive index at a used central wavelength of a light(second polarized light where a y direction orthogonal to the x and zdirections is the polarization direction) which vibrates in the ydirection is defined as n_(l) (n_(h)>n_(l)). In the description below,the refractive index is a refractive index at a used central wavelength.Wavelength ranges of both polarized lights may also be referred to asused wavelengths.

In each embodiment, the optical anisotropic medium is uniaxial. However,in an optical element of another embodiment of the present invention, anoptical anisotropic medium may be biaxial. A refractive index of thelight vibrating in the x and z directions may be n_(l), and a refractiveindex of the light vibrating in the y direction may be n_(h)(n_(h)>n_(l)).

In each embodiment, a difference between the refractive indexes n_(h)and n_(l) is 0.1 or more. Satisfying this condition enables reduction ofa thickness of the first layer 011 to obtain desired birefringencecharacteristics. The small thickness of the first layer 011 enables thefirst layer 011 to function as an interference layer. When therefractive index difference is less than 0.1, since a refractive indexdifference between the polarized lights is small, the thickness has tobe greatly increased to obtain desired birefringence characteristics.

Next, referring to FIGS. 2A and 2B, a refractive index structure of eachembodiment will be described. In FIGS. 2A and 2B, an abscissa axisindicates a thickness of an optical layer (hereinafter referred to as anoptical film thickness) in each of x and y directions, and an ordinateaxis indicates a refractive index of each layer in each of the x and ydirections. A light enters the optical element from the right side ofthe drawing.

Reference numerals 021 and 025 denote refractive indexes (n_(h) andn_(l)) and optical film thicknesses of the first layer 011 in the x andy directions. Reference numeral 022 denotes the same refractive indexand an optical film thickness of the second layer 012 in the x and ydirections, and reference numeral 023 denotes the same refractive indexand an optical film thickness of the third layer 013 in the x and ydirections. Reference numeral 024 denotes the same refractive index andan optical film thickness of the substrate 014 in the x and ydirections. In FIGS. 2A and 2B, “up” and “down” respectively indicate“increase” and “decrease” of refractive index changes at an interfacebetween the layers seen from the opposite side of the substrate 014.

In the first layer 011, refractive indexes are different in the x and ydirections. In the second and third layers 012 and 013 and the substrate014, however, since they are isotropic media, refractive indexes areequal irrespective of directions. Thus, the optical element has anoverall structure where refractive indexes are different in the x and ydirections. The optical film thicknesses of the first layer 011 are alsodifferent according to a ratio of refractive indexes of the x and ydirections.

In the antireflective structure, an antireflective function is obtainedby reflecting a light incident on the substrate on a plurality ofinterfaces, and interfering the reflected lights with each otherconsidered as waves. An interfering method in the antireflectivestructure will be described briefly.

First, an optical element that includes only a substrate will beconsidered. FIG. 16 shows a configuration example of the opticalelement. Reference numeral 161 denotes a substrate made of an isotropicmedium.

FIG. 3 shows a refractive index structure of this optical element. InFIG. 3, an abscissa axis indicates an optical film thickness, and anordinate axis indicates a refractive index. The same applies to thedrawings showing other refractive index structures described below. InFIG. 3, reference numeral 031 denotes a refractive index of thesubstrate 161, and reference numeral 033 denotes an interface betweenthe substrate 161 and an incident medium.

A light is reflected on the interface 033, and amplitude of a wave ofits reflected light is obtained from a refractive index differencebetween the incident medium and the substrate 161. The amplitude isrepresented by Fresnel coefficient, and set to a negative value if arefractive index change at the interface 033 is a refractive indexincrease when seen from the incident side, and to a positive value inthe case of a refractive index decrease. Such a value indicates a phasechange amount of the light reflected on the interface 033.

FIG. 17 shows a configuration example where a thin-film layer isdisposed as a single antireflective layer in the configuration of FIG.16. Reference numeral 171 denotes a substrate, and reference numeral 172denotes a thin-film layer as an antireflective layer. FIGS. 4 and 5 showa refractive index structure of this optical element. Reference numerals041 and 051 denote refractive indexes equal in x and y directions of asubstrate 171, and reference numerals 042 and 052 denote equalrefractive indexes and optical thin films in x and y directions of thethin-film layer 172. Reference numeral 043 denotes an interface betweenthe substrate 171 and the thin-film layer 172, reference numeral 044denotes an interface between the incident medium and the thin-film layer172, reference numeral 053 denotes an interface between the substrate171 and the thin-film layer 172, and reference numeral 054 denotes aninterface between the incident medium and the thin-film layer 172.

FIG. 4 shows a configuration example where a thin-film layer having arefractive index intermediate between those of the substrate and theincident medium is inserted between the substrate and the incidentmedium. Reflection of a light in this configuration is overlapping ofreflection on the interface 043 and reflection on the interface 044. Astate of this overlapped reflection is determined by an optical filmthickness of the thin-film layer 172.

As in the case of the example, when refractive index changes at theinterfaces 043 and 044 of both sides of the thin-film layer 172 are bothincreases, phase changes of waves of reflected lights on both interfaces043 and 044 are similar. A condition for canceling waves with each otheris an optical film thickness of λ/4, and a reinforcing condition is anoptical film thickness of λ/2. In the latter case, a reflected light ofintensity equal to that of the reflected light on the interface 033 ofFIG. 3 is generated. In this example, the case where the refractiveindex changes at the interfaces 043 and 044 are both increases. The sameapplies when the refractive index changes at the interfaces are bothdecreases.

In this configuration, because of an energy conservation law, amplitudeof a reflected light on each interface is not larger than that of thereflected light on the interface 033 of FIG. 3. In view of waveinterferences, only an amount of reflected light equal to that ofreflected light in FIG. 3 is generated even under the reinforcingcondition of the waves. Thus, a total amount of reflected wave in theconfiguration of FIG. 4 is smaller than that in the configuration ofFIG. 3. In other words, the thin-film layer where refractive indexchange directions are similar between both interfaces becomes a layerfor “decreasing the amount of reflected light”.

FIG. 5 shows a configuration example where a thin-film layer having arefractive index higher than those of the substrate and the incidentmedium is inserted between the substrate and the incident medium.

Reflection of a light in this configuration is overlapping of reflectionon the interface 053 and reflection on the interface 054. Thisoverlapping method is determined by an optical film thickness of thethin-film layer 172. When refractive index changes at the interfaces 053and 054 of both sides of the thin-film layer 172 are different, anincrease and a decrease, phase changes of waves of reflected lights onboth interfaces 053 and 054 are reverse. A condition for canceling waveswith each other is an optical film thickness of λ/2, and a reinforcingcondition is an optical film thickness of λ/4. In the former case, areflected light of intensity equal to that of the reflected light on theinterface 033 of FIG. 3 is generated. In this example, the case wherethe refractive index changes at the interfaces are the increase and thedecrease is described. The same applies when the refractive indexchanges at both interfaces are a decrease and an increase.

In this configuration, amplitude of a reflected light on each interfaceis larger than that of the reflected light on the interface 033 of FIG.3. In view of wave interferences, an amount of reflected light equal tothat of reflected light in FIG. 3 is generated even under thereinforcing condition of the waves. Thus, a total amount of reflectedwave in the configuration of FIG. 5 is larger than that in theconfiguration of FIG. 3. In other words, the thin-film layer whererefractive index change directions are different between both interfacesbecomes a layer for “increasing the amount of reflected light”.

An antireflective layer is formed by combining the two types ofthin-film layers in many cases. The simple use of the formerconfiguration leads to a reduction in amount of reflected light.However, a material usable as an antireflective layer is discrete andselective, and a high-performance antireflective function is accordinglydifficult to be obtained only by the former configuration. Thus,combining the former configuration with the latter configuration enablesadjustment of intensity of a partial reflected light, thereby decreasingintensity of the reflected light by the former case. As a result, ahigh-performance antireflective layer (antireflection coating) isobtained.

As described above, in view of wave interferences, the optical filmthicknesses of λ/4 and λ/2 have opposite characteristics. The opticalfilm thickness of “λ/4” can be represented by (2m+1)λ/4 (samecharacteristics are obtained) where m is a natural number, and theoptical film thickness of “λ/2” can be represented by mλ/2.

Based on this theory, to reduce a reflected light for each thin-filmlayer, advisably, an optical film thickness of the thin-film layer isset to a film thickness corresponding to a refractive index change at aninterface between both sides of the thin-film layer. For example, whentwo thin-film layers of refractive indexes sequentially lower withrespect to a refractive index of the substrate are stacked, refractiveindex changes at the interfaces of the thin-film layer are all positive(increase). Therefore, it is preferable that the optical film thicknessof these thin-film layers is set to λ/4. Strictly, intensity of areflected light obtained from a refractive index difference between theinterfaces has to be taken into consideration. But the description isomitted.

When the refractive index structure of the optical element shown in FIG.2 is taken into consideration based on the theory, the optical filmthicknesses 021 and 025 of the first layer 011 are different between thex and y directions. It is because a difference in refractive indexesbetween the directions causes a difference in optical film thicknessbetween the directions even while a physical film thickness is constant.In the case of such an optical element, for example the case of the λ/4plate, the difference between the optical film thicknesses of bothpolarizations is λ/4. Therefore, the optical film thicknesses are λ/4and λ/2 in a polarization direction, resulting in layers of oppositecharacteristics in the antireflective structure.

FIGS. 18A and 18B show a refractive index structure in this case.Reference numerals 181 and 185 denote refractive indexes (n_(h), n_(l))and optical film thicknesses in the x and y directions of the firstlayer 011, and reference numeral 182 denotes equal refractive indexesand optical film thicknesses in the x and y directions of the secondlayer 012 equivalent to the thin-film layer 042 of FIG. 4. Referencenumeral 183 denotes equal refractive indexes and optical filmthicknesses in the x and y directions of a third layer 013 correspondingto the thin-film layer 052 of FIG. 5. Reference numeral 184 denotesequal refractive indexes and optical film thicknesses in the x and ydirections of the substrate 014.

The optical film thickness 181 in the x direction of the first layer 011is 3λ/4, and the optical film thickness 185 in the y direction is λ/2.The refractive index 182 of the second layer 012 is smaller thanrefractive indexes 181 (n_(h)) and 185 (n_(l)) of the first layer 011,and the refractive index 183 of the third layer 013 is set to anintermediate value between the refractive index 184 of the substrate 014and the refractive index 185 (n_(l)) of the first layer 011.

In the configuration of the x direction, refractive index changes of therefractive index 181 of the first layer 011 on both interfaces arereverse, and the layer of the optical film thickness of 3λ/4 is notsuited as an antireflective layer.

In the configuration of the y direction, refractive index changes of therefractive index 185 of the first layer 011 on both interfaces arereverse. However, the configuration is suited as a configuration of anantireflection coating at an optical film thickness of λ/2. In otherwords, since the configuration is ideal in all the layers,antireflective performance is greatly improved.

Thus, to obtain good antireflective functions in both of the x and ydirections, specific conditions are necessary.

The optical element of the embodiment satisfies the following conditionsrepresented by expressions (1) and (2). In these conditionalexpressions, refractive indexes of the first layer at a used centralwavelength λ with respect to first and second polarized lights(x-polarized light and y-polarized light) which enter the opticalelement and whose polarization directions are orthogonal to each otherare n_(h) and n_(l). Refractive indexes of the second and third layersoptically adjacent to the first layer on both sides of the first layerare n_(t1) and n_(t2). The first layer is made of an optical anisotropicmedium, and the second and third layers are made of isotropic media.

The phrase “optically adjacent” includes not only a case where the firstto third layers come into contact with one another (mechanicallyadjacent) at one interface as shown in FIG. 1, but also a case whereinsertion layers extremely thinner than each layer and having limitedoptical influence are held among the first to third layers. In thedescription below, optically adjacent may be simply referred to asadjacent.(n _(t1) −n _(l))·(n _(l) −n _(t2))≦0n_(t1)<n_(l)  (1)(n _(t1) −n _(h))·(n _(h) −n _(t2))≦0n_(t1)>n_(h)  (2)

In the configuration of FIG. 1, the first layer 011 has a refractiveindex (maximum refractive index) n_(h) and a refractive index (minimumrefractive index) n_(l) in the x and y directions. In both sides of thefirst layer 011, the second and third layers 012 and 013 are adjacent tothe first layer 011.

The expressions (1) and (2) indicate that the refractive indexes n_(t1)and n_(t2) of the second and third layers 012 and 013 are both equal tothe minimum refractive index n_(l) or less, or the maximum refractiveindex n_(h) or more of the first layer 011. Satisfying the conditionscauses a refractive index change at the interface of the first layer 011to be in the same direction with respect to both polarized lights, andrefractive index changes at both interfaces of the first layer 011 to bereverse. Thus, the aforementioned “layer for increasing the amount ofreflected light” is configured for both polarized lights. With thislayer configuration, the first layer 011 for both polarized lightsbecomes a layer for adjusting the amount of reflected light in theantireflective layer 015, and a layer for actually reducing the amountof reflected light is the second layer 012 or the third layer 013.

The layer for adjusting the amount of reflected light hascharacteristics that changes of characteristics to an optical filmthickness is more insensitive as compared with the layer for reducingthe amount of reflected light. Thus, the adoption of the configurationenables improvement of reflectance characteristics for both polarizedlights.

If the condition is not satisfied, the refractive index structure of thefirst layer 011 with respect to a polarization directions becomes alayer for “decreasing the amount of reflected light” for at least one ofboth polarized lights. This state is not preferable because reflectioncharacteristics greatly vary from one polarized light to another.

In the embodiment, among the minimum and maximum refractive indexesn_(l) and n_(h) at the used central wavelength λ of the first layer 011,the refractive index having a larger difference from the refractiveindexes n_(t1) and n_(t2) of the second third layers 012 and 013 is n,and a thickness of the first layer 011 is d. In this case, preferably, acondition represented by the following expression (3) is satisfied.

$\begin{matrix}{{0.7{m \cdot \frac{\lambda}{2}}} \leq {n \cdot d} \leq {1.3{m \cdot \frac{\lambda}{2}}\mspace{14mu}( {m\mspace{14mu}{is}\mspace{14mu} a\mspace{14mu}{natural}\mspace{14mu}{number}} )}} & (3)\end{matrix}$

For the condition of the expression (3), the refractive index structureof FIG. 2 will be described as an example. When n_(h)−n_(t1),n_(h)−n_(t2), n_(l)−n_(t1), and n_(l)−n_(t2) are compared with oneanother, a refractive index difference indicated by n_(h)−n_(t1) islargest. Thus, n of the expression (3) becomes n_(h). In other words, inthe refractive index structure of FIG. 2, the optical film thickness 021of the first layer 011 of FIG. 2A is a natural number multiple of λ/2.The adoption of this refractive index structure enables setting of theoptical film thickness 021 of the first layer 011 to λ/2 which is anideal value with respect to a refractive index change.

On the other hand, in the refractive index structure of FIG. 2B, theoptical film thickness 025 of the first layer 011 is not set to an idealvalue for a refractive index change. However, since the first layer 011has small refractive index differences from the second and third layers012 and 013, a refractive index is difficult to increase.

The above refractive index structure is usable for all optical elementswhich use media of high optical anisotropy. For example, in the case ofthe λ/4 plate, an optical film thickness difference between polarizedlights is λ/4. In other words, when one of the optical film thicknessesin the x and y directions of the first layer 011 is set to λ/2, theother optical film thickness becomes λ/4. The antireflective functionaccordingly has opposite characteristics between polarizationdirections. However, setting polarization so that one of the opticalfilm thicknesses in the x and y directions of the first layer 011 can beselectively set to λ/2 enables balancing between polarization of anoptical film thickness of λ/4 and reflection-transmissioncharacteristics.

When the present invention is applied to an optical element which usesan optical anisotropic medium other than the λ/4 plate, an optical filmthickness difference has less influence on antireflective performance ascompared with the λ/4 plate. Thus, in all the optical elements usingoptical anisotropic media, reflection-transmission characteristics canbe improved with respect to both polarized lights.

Next, a case where the optical film thickness 025 of the first layer 011having a refractive index of n_(l) is set to λ/2 will be considered. Inthis case, the refractive index structure shown in FIG. 2B is an idealstructure for refractive index changes. However, in the refractive indexstructure of FIG. 2A, the optical film thickness 021 of the first layer011 is not set to an ideal value for refractive index changes. In therefractive index structure of FIG. 2A, refractive index differences ofthe first layer 011 from the second and third layers 022 and 023 arelarge. As a result, refractive indexes are greatly different between therefractive index structures of FIGS. 2A and 2B.

In the embodiment, an insertion layer may be disposed at least one ofbetween the fist and second layers 011 and 012 and between the first andthird layers 011 and 013. In this case, the expression (1) or (2) isestablished. The insertion layer has a refractive index of n_(h) ormore.

The insertion layer held between the first and second layers 011 and 012is defined as a first insertion layer, and the insertion layer heldbetween the first and third layers 011 and 013 is defined as a secondinsertion layer. When a refractive index and a layer thickness of thefirst insertion layer are respectively n_(o1) and d_(o1) at a usedcentral wavelength, and a refractive index and a layer thickness of thesecond insertion layer are respectively n_(o2) and d_(o2), a conditionrepresented by the following expression (4) is preferably satisfied.

$\begin{matrix}\begin{matrix}{0 \leq {n_{o\; 1} \cdot d_{o\; 1}} \leq \frac{\lambda}{6}} \\{0 \leq {n_{o\; 2} \cdot d_{o\; 2}} \leq \frac{\lambda}{6}}\end{matrix} & (4)\end{matrix}$

FIG. 19 shows a configuration example of an optical element whichincludes an insertion layer. Reference numeral 191 denotes a firstlayer, reference numeral 192 denotes a second layer, and referencenumeral 193 denotes a third layer. Reference numeral 196 denotes aninsertion layer (second insertion layer) disposed between the first andthird layers 191 and 193. The insertion layer 196 is a thin film made ofa homogeneous isotropic medium. Reference numeral 194 denotes asubstrate, and reference numeral 195 denotes an antireflective layer(antireflection coating) which includes the first to third layers 191 to193 and the insertion layer 196.

FIGS. 20A and 20B show the refractive index structure of the opticalelement of FIG. 19. Reference numerals 201 and 205 denote refractiveindexes and optical film thicknesses in the x and y directions of thefirst layer 191, and reference numeral 202 denotes equal refractiveindexes and optical film thicknesses in the x and y direction of thesecond layer 192. Reference numeral 203 denotes equal refractive indexesand optical film thicknesses in the x and y direction of the third layer193, and reference numeral 204 denotes equal refractive indexes andoptical film thicknesses in the x and y directions of the substrate 194.Reference numeral 206 denotes equal refractive indexes and optical filmthicknesses in the x and y direction of the insertion layer 196.

As shown in FIGS. 20A and 20B, the refractive index 206 of the insertionlayer 196 is higher than the refractive indexes n_(h) and n_(l). Thus,this case is outside the above condition for the optical element whichincludes no insertion layer 196. However, an optical film thickness ofthe insertion layer 196 is extremely small as compared with those of theother layers, and thus influence on characteristics of the opticalelement is limited. In other words, even when the insertion layer 196 isdisposed, the first and third layers 191 and 193 can be considered to beoptically adjacent to each other. The same applies when an insertionlayer is disposed between the first and second layers 191 and 192.

An optical film thickness of the insertion layer 196 is preferably setto λ/8 or less, and more preferably λ/10 or less. For the insertionlayer 196, a hard coat layer, an antidazzle layer, or an adhesive layeris used.

In the embodiment, the first layer may be a layer provided withstructural anisotropy realized by forming a plurality of structuressmaller than the used central wavelength λ.

FIG. 6 shows a structure example of the first layer having structuralanisotropy. Reference numeral 061 denotes a substrate, reference numeral062 denotes a first layer having structural anisotropy. The first layer062 includes rectangular lattices made of materials 063 and rectangularlattices made of materials 064 different from the materials 063 whichare alternately and periodically formed in a one-dimensional direction.The lattices made of the materials 063 and 064 constitute one structuresmaller than the used central wavelength λ, and a plurality of thesestructures are formed in the first layer 062. The structures are uniformin the x and z directions, and periodic in the y direction. Referencecode a denotes a width of the lattice made of the material 063,reference code b denotes a width of the lattice made of the material 064(interval between the lattices of the materials 063), and a+b is smallerthan the used central wavelength λ.

In the structure smaller than a wavelength of an incident light, a lightbehaves as if a homogeneous medium is present without being able todirectly recognize the structure. In FIG. 6, the first layer 062functions as a layer having a homogeneous film and an equivalentrefractive index, and has characteristics according to the periodicstructure.

A refractive index n_(x) for a polarized light where the x direction isa polarization direction and a refractive index n_(y) for a polarizedlight where the y direction is a polarization direction in the firstlayer 062 are represented by the following expressions (5) and (6),where n₁ denotes a refractive index of the material 063 and n₂ denotes arefractive index of the material 064.

$\begin{matrix}{n_{x} = \sqrt{\frac{{an}_{1}^{2} + {bn}_{2}^{2}}{a + b}}} & (5) \\{n_{y} = \sqrt{\frac{a + b}{{a/n_{1}^{2}} + {b/n_{2}^{2}}}}} & (6)\end{matrix}$

The expressions (5) and (6) are based on a method called an effectiverefractive index method. This method can obtain a refractive index of astructure anisotropic layer based on a material of the layer and itsfilling rate ff{=a/(a+b)}. Strictly, a structure interval and a usedwavelength have influence, but description thereof is omitted.

To provide the structure anisotropic layer with high anisotropy, settinga large refractive index difference between the materials (materials 063and 064 shown in FIG. 6) of the structure anisotropic layer iseffective. Thus, generally, a material of a high refractive index isused for the material 063, and air is used for the material 064.

For example, when TiO₂ (refractive index 2.3) is used for the material063, and air (refractive index 1.0) is used for the material 064 to seta=b, n_(x)(n_(h)) is set to 1.77, and n_(y)(n_(l)) is set to 1.30,obtaining a very large refractive index difference.

Achieving high anisotropy enables great reduction of a thickness of thestructure anisotropic layer to obtain a desired phase difference. Forexample, when the structure anisotropic layer (first layer) 062 is usedas a λ/4 plate, a thickness is 320 nm at a used central wavelength λ of550 nm.

FIG. 6 shows the case where the structures constituted of therectangular lattices are periodically arrayed in the one-dimensionaldirection. However, the structures may be periodically arrayed in atwo-dimensional or three-dimensional direction. The structures are notlimited to the rectangular lattices, but cylindrical or sphericallattices may be used. If one of the structures is smaller than the usedcentral wavelength, the structures don't have to be arrayedperiodically.

In the embodiment, an inorganic material may be used for a medium havingoptical anisotropy. The inorganic material has high weather resistanceand high heat resistance as compared with an organic material. Using thecharacteristics enables exhibition of sufficient performance even in anoptical element used under severe environment and temperatureconditions.

Hereinafter, specific embodiments will be described with reflectancecharacteristics at design values (experiment values) and an incidentangle 0°. In each embodiment, a used wavelength range is 500 to 600 nm,and a used central wavelength is 550 nm. However, these are onlyexamples, and the embodiments of the present invention are not limitedto these conditions.

Embodiment 1

In Embodiment 1, a substrate having a refractive index of 1.53 isprepared, a thin film (third layer) having a refractive index of 1.63and a physical film thickness (hereinafter simply referred to as filmthickness) of 84 nm is stacked on a surface of the substrate, and anoptical anisotropic layer (first layer) is stacked on a surface of thethin film. A thin film (second layer) having a refractive index of 1.38and a film thickness of 100 nm is stacked on a surface of the opticalanisotropic layer.

Refractive indexes n_(x)(n_(h)) and n_(y)(n_(l)) of the opticalanisotropic layer are respectively 1.92 and 1.80. A physical filmthickness of the optical anisotropic layer is 1108 nm. FIG. 7 showsreflectance characteristics of the embodiment.

Refractive indexes (1.38 and 1.63) of the thin films adjacent to theoptical anisotropic layer satisfy the conditional expression (1).However, the conditional expression (3) is not satisfied. As shown inFIG. 7, reflectances are sufficiently reduced for both polarized lights(x-polarized and y-polarized lights) at a wavelength of 550 nm.

Embodiment 2

In Embodiment 2, a substrate having a refractive index of 1.53 isprepared, a thin film (third layer) having a refractive index of 1.63and a film thickness of 84 nm is stacked on a surface of the substrate,and an optical anisotropic layer (first layer) is stacked on a surfaceof the thin film. A thin film (second layer) having a refractive indexof 1.38 and a film thickness of 100 nm is stacked on a surface of theoptical anisotropic layer. In the optical anisotropic layer, a structurehaving a refractive index of 2.3 is formed into a one-dimensionallattice shape smaller than a used wavelength. A filling rate is 0.85.

Refractive indexes n_(x)(n_(h)) and n_(y)(n_(l)) of the opticalanisotropic layer are respectively 2.16 and 1.80. A physical filmthickness of the optical anisotropic layer is 382 nm. FIG. 8 showsreflectance characteristics of the embodiment.

Refractive indexes (1.38 and 1.63) of the thin films adjacent to theoptical anisotropic layer satisfy the conditional expression (1). Theconditional expression (3) is also satisfied. As shown in FIG. 8,reflectances are sufficiently reduced for both polarized lights at awavelength of 550 nm.

Embodiment 3

In Embodiment 3, a substrate having a refractive index of 2.0 isprepared, a thin film (third layer) having a refractive index of 1.63and a film thickness of 169 nm is stacked on a surface of the substrate,and an optical anisotropic layer (first layer) is stacked on a surfaceof the thin film. A thin film (second layer) having a refractive indexof 1.38 and a film thickness of 100 nm is stacked on a surface of theoptical anisotropic layer. In the optical anisotropic layer, a structurehaving a refractive index of 2.5 is formed into a one-dimensionallattice shape smaller than a used wavelength. A filling rate is 0.9.

Refractive indexes n_(x)(n_(h)) and n_(y)(n_(l)) of the opticalanisotropic layer are respectively 2.40 and 2.00. A physical filmthickness of the optical anisotropic layer is 344 nm. FIG. 9 showsreflectance characteristics of the embodiment.

Refractive indexes (1.38 and 1.63) of the thin films adjacent to theoptical anisotropic layer satisfy the conditional expression (1). Theconditional expression (3) is also satisfied. As shown in FIG. 9,reflectances are sufficiently reduced for both polarized lights at awavelength of 550 nm.

Embodiment 4

In Embodiment 4, a substrate having a refractive index of 1.80 isprepared, a thin film (third layer) having a refractive index of 2.00and a film thickness of 138 nm is stacked on a surface of the substrate,and an optical anisotropic layer (first layer) is stacked on a surfaceof the thin film. A film (second layer) having a refractive index of2.00 and a film thickness of 138 nm is stacked on a surface of theoptical anisotropic layer, and further a thin film having a refractiveindex of 1.38 and a film thickness of 100 nm is stacked on a surface ofthe film. Refractive indexes n_(x)(n_(h)) and n_(y)(n_(l)) of theoptical anisotropic layer are respectively 1.87 and 1.60. A physicalfilm thickness of the optical anisotropic layer is 516 nm. FIG. 10 showsreflectance characteristics of the embodiment.

Refractive indexes (2.00 and 2.00) of the thin films adjacent to theoptical anisotropic layer satisfy the conditional expression (2). Theconditional expression (3) is also satisfied. As shown in FIG. 10,reflectances are set to minimal values at a wavelength of 550 nm forboth polarized lights. In other words, reflectances are sufficientlyreduced for both polarized lights at the wavelength of 550 nm.

Embodiment 5

In Embodiment 5, a substrate having a refractive index of 2.0 isprepared, a thin film (third layer) having a refractive index of 1.63and a film thickness of 169 nm is stacked on a surface of the substrate,a thin film (second insertion layer) having a refractive index of 2.30and a film thickness of 10 nm is stacked on a surface of the thin film(the third layer), and an optical anisotropic layer (first layer) isstacked on a surface of the thin film (the second insertion layer). Athin film (second layer) having a refractive index of 1.38 and a filmthickness of 100 nm is stacked on a surface of the optical anisotropiclayer. Refractive indexes n_(x)(n_(h)) and n_(y)(n_(l)) of the opticalanisotropic layer are respectively 2.41. and 2.00. A physical filmthickness of the optical anisotropic layer is 335 nm. FIG. 11 showsreflectance characteristics of the embodiment.

In the thin film having a film thickness of 10 nm, an optical filmthickness (n_(o2)·d_(o2)) calculated by the expression (4) is anextremely small value of 23 nm. The optical anisotropic layer canaccordingly be considered to be optically adjacent to the thin filmhaving a refractive index of 1.63. Thus, refractive indexes (1.63 and1.38) of the thin films adjacent to the optical anisotropic layersatisfy the conditional expression (1). The conditional expression (3)is also satisfied. As shown in FIG. 11, reflectances are set to minimalvalues at a wavelength of 550 nm for both polarized lights. In otherwords, reflectances are sufficiently reduced for both polarized lightsat the wavelength of 550 nm.

Table 1 collectively shows whether the numerical values and theconditions of Embodiments 1 to are satisfied (o). In Table 1, *1indicates calculation carried out by assuming that it is the thin-filmlayer (third layer) having a refractive index of 1.63 and a filmthickness of 169 nm that is optically adjacent to the opticalanisotropic layer based on the expression (4).

TABLE 1 Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Embodiment 5Physical Physical Physical Physical Physical Refractive film Refractivefilm Refractive film Refractive film Refractive film # index thicknessindex thickness index thickness index thickness index thickness Incident1.00 1.00 1.00 1.00 1.00 medium Thin-film 1.38 100 1.38 100 1.38 1001.38 100 1.38 100 layer 1 Thin-film — — — — — — 2.00 138 — — layer 2Optical 1.92 1108  2.16 382 2.40 344 1.87 516 2.40 353 Anisotropic 1.801.80 2.00 1.60 2.00 Layer Insertion — — — — — — — — 2.30  10 layerThin-film 1.63 84 1.63  84 1.63 169 2.00 138 1.63 169 layer 3 Substrate1.53 1.53 2.00 1.80 2.00 Conditions ◯ ◯ ◯ ◯ ◯ *1 (1) and (2) Condition7.75λ/2 ◯ 3λ/2 ◯ 3λ/2 ◯ 3λ/2 ◯ 3.1λ/2 ◯ *1 (3) Condition — — — —   λ/25◯ (4)

Hereinafter, Comparative Examples 1 and 2 corresponding to Embodiments 1to 5 will be described. These Comparative Examples do not satisfy (x)conditions (1) and (2) of Table 2.

Comparative Example 1

In Comparative Example 1, a substrate having a refractive index of 1.80is prepared, and an optical anisotropic layer (first layer) is formed ona surface of the substrate. A thin film having a refractive index of1.38 and a physical film thickness of 100 nm is stacked on a surface ofthe optical anisotropic layer. In the optical anisotropic layer, astructure having a refractive index of 1.8 is formed intoone-dimensional lattice shape smaller than a used wavelength. A fillingrate is 0.80. Refractive indexes n_(x) and n_(y) of the opticalanisotropic layer are respectively 1.67 and 1.50. A physical filmthickness of the optical anisotropic layer is 825 nm. FIG. 12 showsreflectance characteristics of Comparative Example 1.

In Comparative Example 1, to prevent reflection, the thin films havingthe low refractive indexes are stacked in the upper part. However, sincea relationship of the refractive indexes or the film thicknesses are notappropriate, reflectances (especially reflectances at a wavelength of550 nm) are greatly different between both polarized lights. Thus,Comparative Example 1 is not preferable.

Comparative Example 2

In Comparative Example 2, a substrate having a refractive index of 1.53is prepared, a thin film having a refractive index of 2.30 and a filmthickness of 120 nm is stacked on a surface of the substrate, and anoptical anisotropic layer (first layer) is formed on a surface of thethin film. A thin film having a refractive index of 1.38 and a filmthickness of 100 nm is stacked on a surface of the optical anisotropiclayer. In the optical anisotropic layer, a structure having a refractiveindex of 2.3 is formed into one-dimensional lattice shape smaller than aused wavelength. A filling rate is 0.80. Refractive indexes n_(x) andn_(y) of the optical anisotropic layer are respectively 2.11 and 1.69. Aphysical film thickness of the optical anisotropic layer is 325 nm. FIG.13 shows reflectance characteristics of Comparative Example 2.

In Comparative Example 2, the thin-films are stacked on both interfacesof the optical anisotropic layer. However, no antireflection function isprovided.

TABLE 2 Comparative Comparative Example 1 Example 2 Refractive Physicalfilm Refractive Physical film # index thickness index thickness Incidentmedium 1.00 — 1.00 — Thin-film layer 1 1.38 100 1.38 100 Thin-film layer2 — — — — Optical anisotropic 1.67 825 2.11 325 layer 1.50 1.69Insertion Layer — — — — Thin-film layer 3 — — 2.30 120 Substrate 1.80 —1.53 — Conditions(1) and X X (2) Condition(3) — — Condition(4) — —

Embodiment 6

Hereinafter, a liquid crystal projector and an optical pickup apparatuswill be described as examples of optical apparatus which use the opticalelements of Embodiments 1 to 5. However, the optical elements ofEmbodiments can be used for other optical apparatus.

FIG. 14 shows a configuration of a liquid crystal projector. Referencenumeral 140 denotes a light source (lamp), reference numeral 141 rdenotes an optical path of a red light, reference numeral 141 g denotesan optical path of a green light, and a reference numeral 141 b denotesan optical path of a blue light. Reference numeral 142 denotes apolarization conversion element, and reference numeral 143 denotes adichroic mirror. Reference numeral 144 denotes a polarization plate, andreference numeral 145 denotes a wavelength-selective phase plate.

Reference numeral 146 i denotes a green polarization beam splitter, andreference numeral 146 p denotes a red polarization beam splitter.Reference numerals 147 r, 147 g, and 147 b respectively denote red,green and blue 1/4λ plate. Reference numerals 148 r, 148 g, and 148 brespectively denote red, green, and blue reflective liquid crystalpanels (image forming elements). Reference numeral 149 denotes a colorsynthesis prism, and reference numeral 1401 denotes a projection lens.

A beam from the light source 140 is converted into a beam having aspecific polarization direction by the polarization conversion element142. The polarized beam is converted for each color into a P-polarizedor S-polarized light by the wavelength-selective phase plate 145 and1/4λ plates 147 r, 147 g and 147 b, and reflected or transmitted by thepolarization beam splitters 146 i and 146 p. Thus, each color lightprojected through a corresponding optical path to a projected surfacesuch as a screen (not shown) by the projection lens 1401 to form a colorimage on the projected surface.

The optical elements of Embodiments 1 to 5 can be used as thewavelength-selective phase plate 145 and the 1/4λ plates (phase plates)147 r, 147 g, and 147 b.

The optical elements of Embodiments 1 to 5 can be miniaturized sincetheir thicknesses can be reduced, and high transmittances can berealized as the phase plates for entering P-polarized and S-polarizedlights. As a result, a high-performance projector can be provided. Theuse of the inorganic material for the first layer of the optical elementenables realization of a projector of high heat resistance and highweather resistance.

FIG. 15 shows an optical system around a light source in an optical diskoptical pickup apparatus. Reference numeral 151 denotes a light source(laser), and reference numeral 151 a denotes a polarization plate.Reference numeral 152 denotes a polarization beam splitter, andreference numerals 153 and 155 denote condenser lenses, and referencenumerals 154 and 156 denote photodetectors. Reference numeral 157denotes an optical disk, and reference numeral 158 denotes a phaseplate.

A beam from the light source 151 enters the polarization beams splitter152 as a beam (P-polarized light) having a specific polarizationdirection by the polarization plate 151 a. A part of the polarized beamis reflected by the polarization beam splitter 152 to be monitored bythe photodetector 154 via the condenser lens 153.

The polarized beam transmitted through the polarization beam splitter152 forms an image on the optical disk 157 by the condenser lens 155 viathe phase plate 158. The reflected light optically modulated at theoptical disk 157 has its polarization direction rotated by 90° by thephase plate 158 to become an S-polarized light, and is reflected by thepolarization beam splitter 152. Then, the reflected light is detected bythe photodetector 156 for detecting a reproduced signal light.

The optical elements of Embodiments 1 to 5 can be used for the phaseplate 158. The optical elements of Embodiments 1 to 5 can beminiaturized because their thicknesses can be reduced, and hightransmittances can be realized for entering P-polarized and S-polarizedlights. As a result, a high-performance optical pickup apparatus can beprovided. The use of the inorganic layer for the first layer of theoptical element enables realization of an optical pickup apparatus ofhigh heat resistance and high weather resistance.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2008-139360, filed on May 28, 2008, which is hereby incorporated byreference herein in its entirety.

1. An optical element comprising: a first layer made of a medium havingoptical anisotropy, wherein a difference between refractive indexesn_(h) and n_(l) (n_(h)>n_(l)) at a central wavelength λ for first andsecond polarized lights which enter the optical element and whosepolarization directions are different from each other is at least 0.1,and wherein conditions below are satisfied, where n_(t1) and n_(t2)denote refractive indexes of second and third layers optically adjacentto the first layer in both sides of the first layer and made ofisotropic media at the central wavelength:(n _(t1) −n _(l))·(n _(l) −n _(t2))≦0n_(t1)<n_(l).
 2. An optical element according to claim 1, wherein acondition below is satisfied, where n denotes a refractive index of therefractive indexes n_(h) and n₁ larger in difference from the refractiveindexes n_(t1) and n_(t2) of the second and third layers, and d denotesa thickness of the first layer:${0.7{m \cdot \frac{\lambda}{2}}} \leq {n \cdot d} \leq {1.3{m \cdot \frac{\lambda}{2}}\mspace{14mu}( {m\mspace{14mu}{is}\mspace{14mu} a\mspace{14mu}{natural}\mspace{14mu}{number}} )}$3. An optical element according to claim 1, further comprising at leastone of a first insertion layer disposed between the first and secondlayers and a second insertion layer disposed between the first and thirdlayers, wherein conditions below are satisfied, where at the centralwavelength, n_(o1) denotes a refractive index and d_(o1) denotes a layerthickness of the first insertion layer, and n_(o2) denotes a refractiveindex and d_(o2) denotes a layer thickness of the second insertionlayer: $\begin{matrix}{0 \leq {n_{o\; 1} \cdot d_{o\; 1}} \leq \frac{\lambda}{6}} \\{0 \leq {n_{o\; 2} \cdot d_{o\; 2}} \leq \frac{\lambda}{6}}\end{matrix}$
 4. An optical element according to claim 1, wherein thefirst layer is a layer having structural anisotropy obtained by forminga plurality of structures smaller than the central wavelength.
 5. Anoptical element according to claim 1, wherein the first layer is made ofan inorganic material.
 6. The optical element according to claim 1,wherein the optical element is a phase plate.
 7. An optical apparatuscomprising: an optical element; wherein the optical element includes afirst layer made of a medium having optical anisotropy, wherein adifference between refractive indexes n_(h) and n_(l) (n_(h)>n_(l)) at acentral wavelength λ for first and second polarized lights which enterthe optical element and whose polarization directions are different fromeach other is at least 0.1, and wherein conditions below are satisfied,where n_(t1) and n_(t2) denote refractive indexes of second and thirdlayers optically adjacent to the first layer in both sides of the firstlayer and made of isotropic media at the central wavelength:(n _(t1) −n _(l))·(n _(l) −n _(t2))≦0n_(t1)<n_(l).